The goal is to understand how experiential information becomes integrated into the functional organization of the mammalian brain. One likely mechanism involves alteration of synaptic patterns. In adults, this may occur through active formation of synapses in response to experience, whereas in early development, selective retention of part of a population of constitutively overproduced synapses may be involved. Housing of animals under conditions of differential environmental complexity, either at weaning or in adulthood, changes the number and the structure of forebrain synapses. This procedure will be used to test the hypothesis that active synaptogenesis underlies information storage. Preliminary anatomical data suggest stable experience effects in occipital cortex, whereas electrophysiological data suggest transient effects in hippocampus. Objectives are to 1) examine early morphological phases of the occipital cortical and hippocampal response to differential environmental complexity, when active synaptogenesis should be most easily distinguishable from constitutive turnover and synapse maturation effects will be minimized; 2) study the permanence of these morphological changes to determine whether they, like memories, are stable without continued differential experience and whether polyribosomal aggregate (PRA) location, which appears to indicate synaptogenesis, returns to a baseline state; 3) determine whether experience-induced increases in the hippocampal dentate gyrus response to afferent activity increase, reduce, or do not affect subsequent long term potentation, to assess whether these effects share a common mechanism; 4) study electrophysiological correlates of experience-induced differences in occipital cortical synaptic connectivity, to assess their role in brain function; and 5) to attempt to purify the mRNA associated with PRA located postsynaptically during synaptogenesis, to further understand synapse formation and its role in development and memory. Morphological procedures involve stereological analysis of electron and light microscopic data. Electrophysiological studies use isolated CNS preparations in vitro. Biochemical techniques involve density gradient centrifugation and affinity chromatography. Understanding mechanisms underlying developmental information storage and memory is essential to developing clinical treatments for learning and memory dysfunction, mental retardation, and related mental health problems.